Drill Master: Understanding Bend Radius

One of the most important aspects of any HDD project is the planning that goes on before the job is started. More specifically, determining the entry and exit points, the depth that must be achieved and direction changes for the drill path, are all major parts of the planning process. One of the keys to these calculations is bend radius.

Bend radius is a term that many people use, but few understand. Often times it is confused with “the number of feet needed to make a 90-degree turn.” Understanding bend radius and how to use it to maximize profits and minimize downtime is of the utmost importance to the driller. It is also important to remember that bend radius applies not only to the pipe in the drill string, but also mud motors, wireline systems, and the product pipe being pulled in when completing the job.

Bend radius by definition is the forward distance required for a drill string to make a 90-degree turn (see figure on pg. 61). In practicality, bend radius is an indication of how much bending a pipe can take without doing significant damage to the drill stem. In the past, drill pipe manufacturers used different methods to establish the bend radius rating for the their pipe. In June 2000, SAE standard J2022 was established so that all drill pipe could be evaluated using the same baseline. Since use of the SAE guidelines is voluntary, it is necessary to know what method of calculation is being used by a manufacturer when comparing the bend radii of drill pipe from different manufacturers.

It is important to realize that drill pipe wear and its ultimate failure is a fact of life. The only way not to wear out drill pipe is to not use it. However, if we factor in bend radius when plotting the drill path and doing the actual job, the useful life of the drill pipe can be optimized.

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How can drillers use bend radius information when doing a job? The first step is to get the drilling rig operator’s manual and study the bend radius numbers for the drill pipe. There should be bend radius recommendations in the book stating what steering change can be made for each stem of pipe that is drilled in. Remember that the allowable steering change for each pipe applies to both up/down pitch changes, as well as left/right corrections. Using the bend radius information helps determine the amount of set-back needed by the drill unit at the entry point. The shallower the level off depth of the bore, the less the entry angle should be and the farther back the unit must be set up. Vice-versa, the deeper the level off depth, the steeper the entry angle can be.

During the bore, any changes and corrections that are made should not exceed the recommended bend radius of the drill pipe. The worst-case scenario for any job is that the drill path ends up looking like a “V” with a steep bottom rather than a smooth curve with no abrupt changes. The same holds true for the exit pit. There should be no quick changes and no “turn it to 12 and push Ôtil you pop out.”

How does bend radius relate to the product pipe that is being pulled in? In general, the ratings for plastic, PVC, and HDPE pipe will not exceed that of the drill pipe.

The exception may be the larger diameters of these types of pipe where the integrity of the pipe and the joints becomes a concern if they are pulled through severe bends.

When pulling back steel pipe, it is essential the bend radius of that pipe be considered when developing the bore plan. For planning purposes, allow approximately 100 ft of bend radius for every inch of diameter for steel pipe. In other words, for a 3-in. steel pipe use a 300-ft bend radius for planning and for 12-in. steel pipe, allow 1,200 ft for the bend radius calculations. As can be seen, the steel product pipe usually has a much larger bend radius than the drill pipe. The largest bend radius of either the drill pipe or product pipe is the number that must be used for planning and completing the bore.

What is the penalty for exceeding the bend radius of the drill pipe? From a monetary standpoint, it can be significant. When the bend radius is exceeded, the useful life of the drill pipe decreases dramatically. The damage that is done to the pipe is subtle and most likely will not be noticed immediately. The failure and breaking of the pipe usually occurs several jobs after the damage is done.

Many times the break happens on a simple bore, and no one can figure out what happened to cause the pipe to fail. Too often, the job that caused the failure is long forgotten and never factored into the premature failure of the drill pipe. So what happens when there is a job requiring the bend limits of the drill pipe be exceeded? Contractor have two options. They could walk away from the job because the damage to the drill stem will be excessive and costly.

The other option is to bid the job, factoring in the damage to the drill pipe and the increased cost of replacing the drill string sooner than would normally be expected.

When the bend radius of the product pipe is exceeded, there are consequences to deal with also. The primary issue is a harder than expected pullback. The product pipe must go through tight bends and curves, and there is a much greater chance of hanging up downhole. If the drill path is smooth and gradual, and the tail pit is of adequate length based on the product pipe’s bend radius, the pullback will be much easier and quicker, which equates to less expense and more profits.

Again, a monetary gain when properly factoring in bend radius for the job. The final area of concern involves the use of mud motors and wireline systems. The mud motor itself is a very stiff section, and depending upon the length, generally has a bend radius greater than the drill string. The stainless steel monel section that houses the steering tool is also very stiff and more brittle than the drill string.

The suppliers of the mud motor and steering tools should be able to provide information and recommendations about what bend radius number should be factored into the bore plan.

There are many unknowns associated with underground construction jobs. Any information the driller can factor into the drilling equation, increases the chances of success and profitability. Bend radius is one of the factors that should be utilized when analyzing and pre-planning for any HDD project. The benefits are quicker pullbacks, fewer failed bores, less downtime and more life out of the drill string. All of these benefits add up to what it’s all about — increased profits.